As a sidenote, I suggest you use some other email client if you can. Consider Mozilla Thunderbird, for instance. Outlook isn't the best on security, filtering and stability -- and that's putting it lightly.

For yahoo, if you have a premium account then you can configure it to work with thunderbird with ease. However, if you have a free account, then you will need a program like yahoopops, or something similiar. Here is a thread for thunderbird that has links to some of the program websites. http://forums.mozillazine.org/viewtopic.php?t=53174